The operating point in the V-I plane (input voltage-current) of an energy storage-type DC-to-DC converter during start up follows a particular trajectory, until an equilibrium or steady-state operating point locus is attained, at which the converter operating point is stabilized. This operating point lies on a locus of steady-state operating points upon which the converters output is regulated. If the power source energizing the converter is current-limited, such as may be the case when converters are cascaded, or power is supplied through a long transmission line, conditions due to loading circuit components may be such that the source output current characteristic, combined with the start-up trajectory of the converter, defines an equilibrium operating point off the curve of steady-state operating points, so that the converter is unable to fully turn on to supply its regulated output. A detailed discussion of the start-up phenomena may be found in a published paper, by T. G. Wilson, Jr., entitled "Start-Up Transient of a DC-to-DC Converter Powered By a Current-Limited Source," and appearing in IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-17, No. 3, May 1981, pages 351-363.
If, as indicated in the cited reference, the source powering the DC-to-DC converter is unable to supply the needed current level, the converter fails to reach an equilibrium operating point at which its regulated output voltage can be supplied and, hence, does not properly startup. Solutions in the prior art to counteract this tendency to not properly start have included soft-start techniques, which involve gradually turning on the converter by timing techniques that slow the rate of rise of the output voltage, and techniques operating the converter power switch at a fixed maximum duty cycle during start up until a preset output voltage threshold is reached. This soft-start solution is very time dependent; that is, it has a fixed duration of operation every time the converter is turned on and is, hence, an unsatisfactory arrangement if the converter is to be used in situations where required time constants may vary due to the source current capabilities varying or input or output capacitance of the converter varying. The technique of fixing a maximum duty cycle of the power switch is disadvantageously very sensitive to changes in the value of the fixed maximum duty cycle value wherein a very small shift in the duty cycle induces large changes in the converter characteristic as examined in the input current vs. input voltage plane.
Another prior art approach is to use a start-up circuit with a breakdown device that prohibits the power switch from conducting until a breakdown voltage level is attained at the input. That has the effect of horizontally shifting the start-up trajectory to vertically lower the intersection of the start-up ascent trace and the locus of steady-state operating points, so that the operating point may reach its desired equilibrium operating point. However, this technique does not work in all situations even though it modifies the static input voltage vs. input current characteristic of the converter such that the input current of the converter never exceeds the source current. The dynamic characteristics may still allow the input current to exceed the source characteristics. This may prevent the converter from properly starting.